1. Field of the Invention:
The present invention relates to a fabricating method for forming a micro lens of compound semiconductor utilized in a field of an optical information processing system or an optical measurement system.
2. Description of the Prior Art:
In an opto-electrical integrated circuit used in an optical information processing system or an optical communication, or an optical measurement etc., it is required to fabricate a substrate integrally with various optical components formed thereon. For example, a semiconductor laser as a light-emitting device and a micro lens are required to be installed together on the same substrate surface in order to minimize optical elements and obtain high performance.
A conventional fabricating method for forming a micro lens on a compound semiconductor substrate is, for example, disclosed in the Applied Physics Letters 55 (2) (1989) pp. 97-99.
FIG. 6 is a schematic view showing a process of such a conventional fabricating method for forming a micro lens on a compound semiconductor substrate. This conventional fabricating method of the micro lens is as follows.
Photolithography and etching method using the bromine:methanol etchants are alternately applied on a GaP substrate 61 by successively increasing size of circular mask 62. With these processes, the GaP substrate 61 is stepwisely etched so as to accumulate a plurality of coaxial steps, constituting a rough outline of a micro lens as shown in FIG. 6(d). Subsequently, thus etched substrate is exposed in an atmosphere of phosphine (PH.sub.3) and hydrogen (H.sub.2) gases at approximately 1000.degree. C. so that the mass transport can be carried out. During several tens hours to 100 hours, stepped portions of the GaP substrate are gradually smoothened. And finally, as shown in FIG. 6(e), a spherical lens 63 can be formed.
However, above conventional fabricating method of a micro lens is disadvantageous in that photolithography and etching method must be alternately carried out to form coaxial etching steps on the compound semiconductor substrate. Furthermore, it is feared that a high-temperature process required for executing the mass transport may induce a sever property change or a significant deterioration of electronic devices or optical devices fabricated in advance on the substrate.